This invention relates to web coating, and particularly to a method and apparatus for applying a coating composition to a passing web or webs under controlled conditions which include close containment of the coating composition up to the point of application and wiping of the coated surface of the web by a countermoving surface as the web leaves the point of application.
Various coating means of this general nature have previously been proposed, as in U.S. Pat. Nos. 3,235,400 and 4,294,872, but the present invention provides an alternative that is effective and overcomes disadvantages of prior proposals.
The invention has various possible applications, and presently appears particularly useful for coating webs of paper or plastic with highly reactive solventless coating compositions, such as the catalyzed silicone compositions referred to in U.S Pat. No. 4,256,870. As discussed in that patent, such compositions are characterized by a very short pot life in the absence of the use of a cure inhibitor which delays cure until the temperature of the coating is elevated in a curing oven or the like. Such coatings are so reactive that no means of coating has been fully satisfactory without use of the inhibitor and a subsequent heat cure. Apparatus is available to accurately meter and quickly mix separately stored components of such coating compositions, particularly the base polymer, catalyst, and cross linker, but with the short pot life of such mixture in the absence of an inhibitor, there has been no way to apply the coating composition without use of the inhibitor and subsequent heat cure. The present invention provides such a means, or at least minimizes reliance on heat cure. The result is a means for reducing or possibly eliminating heat treatment with attendant savings in the cost of providing and operating some or all of the curing oven capacity which is presently used.
Silicone coatings are generally used as release compositions to enable pressure-sensitive adhesive tapes or sheets or the like to be easily pulled from a protective liner or backing, or to be unwound in self-wound constructions, without degradation of the adhesive quality of the construction. Silicone coatings with no volatile components require light coating weights. The coatings must be reasonably uniform, and the combination of uniformity and light coating weight makes a demanding coating requirement. The present invention meets this requirement.
Silicone coatings, and particularly highly reactive coatings, are mentioned because they exemplify demanding coating applications, one example being moisture cure silicones such as room temperature vulcanization type compounds. However, the invention also provides effective coating means for coatings other than silicone compositions, e.g., urethanes that are moisture-curing. Also, the invention may be used for coatings that are neither reactive nor light in coating weight.